A peek inside Bill Gross’ stamp collection that he’s selling

The historic Pony Express cover signed by co-founder William H. Russell estimated to fetch between $75,000 and $100,000 at auction.

He also has a block of six of the first U.S. stamps, which were issued in 1847, worth between $500,000 and $750,000.

Charles Shreve, director for Robert A. Siegel Auction Galleries and Gross’ stamp advisor for the past 25 years, told CNBC that block of stamps somehow ended up in a Bible, where they remained for “dozens and dozens of years.” It remains the only intact block of six of the first U.S. stamps, he said.

Also included in the collection is a set of upside-down steamship stamps, the largest known intact multiple of any inverted stamp error, said Shreve. “Stamp collectors love errors,” he said.